giant star

A giant star is a star much larger than a main sequence star, with a radius on the order that of a solar system planet's orbit (e.g., an astronomical unit). After their main sequence phase, many stars go through phases in which they become giants, e.g., the red-giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB). The term was coined for stars especially bright (i.e., with a large luminosity) for their temperature. Hotter stars are brighter given their size, but at a given temperature, a star's brightness is directly related to the area facing us (a multiple of the square of the radius). The general term dwarf star actually applies to any star that is not a giant (e.g., the Sun), though often "dwarf" is used with a qualifier to signify a more specific meaning (white dwarf, red dwarf, brown dwarf).

Stars grow to be giants if their luminosity is high and their mass is low, i.e., a lower mass than that of a main-sequence star of the same luminosity: a giant star's luminosity is often roughly that of (main-sequence) B-type stars. Such high luminosity (energy production) occurs in some post-main-sequence phases due to larger volumes supporting the conditions to produce fusion, and/or the occurrence of additional fusion reactions due to the star developing their necessary density, temperature and fuel. Their low mass allows the star to "puff up", and given the resulting much larger surface, the energy passed from within is diluted further as it moves outward and the photosphere temperature is cooler, resulting in a more-reddish color. (A main-sequence star with the same luminosity has its larger mass's gravity, which keeps the star more compact.) The ultimate large size of the star is due to the balance of forces/mass, including the heat of the extensive fusion, and is a bit non-linear because gas pushed further from center, in turn has less pull from the star's gravity, which follows an inverse square law. A factor is the outward radiation pressure: radiative transfer is a net outward movement of photons, producing a net outward force.

Giant stars are of various colors, red (red giants) being most common, but can be blue (blue giants) or yellow (yellow giants), and the term white giant is sometimes used for stars between the two in temperature. The largest are red: a smaller giant (subgiant) may simply be a less bright star, or may find its balance at a somewhat smaller size, their luminosity producing a higher temperature, making them less red. These alternate colors can occur during different post-main-sequence phases, with other characteristics of the star being additional deciding factors, including the star's mass and metallicity. Especially large giants are known as bright giants or for even brighter, supergiants, then hypergiants.

It is very clear that the expansion of a star to become a giant often results in planetary engulfment and some material that was much earlier in the protoplanetary disk finally becomes part of the star. This may leave signatures that can be detected in giant stars or subsequent stellar remnants. My own logic tells me that the most likely random collisions (including actual contact) of stars (stellar collisions, e.g., within globular clusters) are those involving a giant. Also, there must exist some stars that passed through the outer portion of a giant star during their history, which could conceivably cause peculiarities.